Abnormality diagnosis apparatus for particulate filter

ABSTRACT

Embodiments of the present disclosure may improve the accuracy of abnormality diagnosis of a particulate filter using the output value of a particulate matter (PM) sensor provided in an exhaust passage. A time when deposition of PM between electrodes of the PM sensor is restarted after completion of a sensor recovery process may be specified as a PM deposition restart time. A process of diagnosing an abnormality of a filter based on the output value of the PM sensor at a first determination time after the PM deposition restart time may be specified as a filter diagnosis process. To determine, whether the filter diagnosis process is to be performed, a change rate of the output value of the PM sensor in a time period from the PM deposition restart time to a predetermined second determination time before the first determination time, may be compared with a threshold value.

This nonprovisional patent application is based on and claims thebenefit of Japanese Patent Application No. 2015-076057 filed on Apr. 2,2015, the entire contents of which are hereby incorporated by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to an abnormality diagnosis apparatus fora particulate filter that is provided in an exhaust passage of aninternal combustion engine to trap PM (particulate matter) included inthe exhaust gas.

2. Description of Background Art

There is a known technique to provide a particulate filter (whichhereinafter may be simply referred to as “filter”) that is configured totrap PM included in the exhaust gas, in an exhaust passage of aninternal combustion engine. However, in such a technique, failure suchas erosion or breakage may occur in the filter. The occurrence of such afailure increases the amount of PM that is not trapped by the filter butflows out of the filter. The occurrence of such a failure in the filteror the occurrence of an abnormality of the filter, for example,detachment of the filter from the exhaust passage, leads to an increasein PM released to the atmosphere. A proposed technique accordinglyprovides a PM sensor downstream of the filter in the exhaust passage anddiagnoses an abnormality of the filter based on the output value of thePM sensor. A known configuration of the PM sensor used for abnormalitydiagnosis of the filter has a pair of electrodes as a sensor element andoutputs a signal corresponding to the amount of PM depositing betweenthe electrodes.

A technique disclosed in Patent Literature 1 compares an output value ofa PM sensor provided downstream of a filter in an exhaust passage withan estimated value of a deposition amount of PM in the PM sensor todetermine the presence or the absence of a failure in the filter. Thetechnique described in Patent Literature 1 estimates a flow-out amountof PM from the filter on the assumption that the filter is in apredetermined state, and calculates an estimated value of the depositionamount of PM in the PM sensor based on the integrated value of theestimated flow-out amount of PM. The state of the filter may be detectedby comparing the estimated value of the deposition amount of PM with theactual output value of the PM sensor.

PATENT LITERATURE

-   Patent Literature 1: Japanese Patent Application Laid-Open No.    2011-179467A.

SUMMARY Technical Problem

As described above, the occurrence of a failure in the filter canincrease the flow-out amount of PM from the filter. This can lead to anincrease in amount of PM trapped between the electrodes of the PM sensorthat is provided downstream of the filter in the exhaust passage. Thismay provide the larger deposition amount of PM between the electrodes ofthe PM sensor, compared with that deposition amount of PM in the normalstate of the filter. The same applies to the case where the filter isdetached from the exhaust passage. Abnormality diagnosis of the filtercan thus be performed, based on the output value of the PM sensor in apredetermined time period.

There is a likelihood that conductive materials other than PM as theoriginal detection object (hereinafter referred to as extraneoussubstances) are trapped between the electrodes of the PM sensor. Even inthe normal state of the filter, such extraneous substances may betrapped between the electrodes of the PM sensor. Trapping the extraneoussubstances between the electrodes of the PM sensor also varies theoutput value of the PM sensor. In the case of abnormality diagnosis ofthe filter based on the output value of the PM sensor in the state thatextraneous substances are trapped between the electrodes of the PMsensor, there is a likelihood of wrong diagnosis that the filter isabnormal, despite that the filter is actually normal.

Embodiments of the present disclosure may improve the accuracy ofabnormality diagnosis of a filter using the output value of a PM sensorprovided downstream of the filter in an exhaust passage.

According to an aspect of the present disclosure, a time when depositionof PM between electrodes of the PM sensor is restarted after completionof a sensor recovery process may be specified as a PM deposition restarttime. A process of diagnosing an abnormality of a filter based on theoutput value of the PM sensor at a first determination time after the PMdeposition restart time may be specified as a filter diagnosis process.It can be determined whether the filter diagnosis process is to beperformed or not by comparing a change rate of the output value of thePM sensor in a time period from the PM deposition restart time to apredetermined second determination time before the first determinationtime with a threshold value.

More specifically, according to one aspect of the present disclosure,there may be provided an abnormality diagnosis apparatus for aparticulate filter that is provided in an exhaust passage of an internalcombustion engine to trap PM included in exhaust gas. The abnormalitydiagnosis apparatus may comprise: a PM sensor that is provideddownstream of the particulate filter in the exhaust passage and isconfigured to have a pair of electrodes as a sensor element and output asignal corresponding to a deposition amount of PM between the electrodeswhen electrical continuity is established between the electrodes bydeposition of PM between the electrodes, the PM sensor being configuredsuch that a larger deposition amount of PM between the electrodesprovides a higher variation in output value of the PM sensor relative toan increase in deposition amount of PM between the electrodes; acontroller comprising at least one processor configured to perform asensor recovery process and a filter diagnosis process, the sensorrecovery process being a process of removing PM depositing between theelectrodes of the PM sensor, the filter diagnosis process being aprocess of diagnosing an abnormality of the particulate filter based onan output value of the PM sensor at a predetermined first determinationtime after a predetermined PM deposition restart time that is a timewhen deposition of PM between the electrodes of the PM sensor isrestarted after completion of the sensor recovery process; and a monitorunit that is configured to continuously monitor an output signal of thePM sensor after the PM deposition restart time. An estimated value ofthe deposition amount of PM between the electrodes of the PM sensor onthe assumption that the particulate filter is in a predeterminedreference state may be specified as a reference deposition amount of PM.A variation in output value of the PM sensor monitored by the monitorunit per unit increase in reference deposition amount of PM or avariation in output value of the PM sensor monitored by the monitor unitper unit time may be specified as a sensor output change rate. When thesensor output change rate becomes higher than a predetermineddetermination change rate in a time period from the PM depositionrestart time to a predetermined second determination time prior to thefirst determination time, the filter diagnosis process by the controllermay not be performed. When the sensor output change rate does not becomehigher than the predetermined determination change rate in the timeperiod from the PM deposition restart time to the second determinationtime, the filter diagnosis process by the controller may be performedirrespective of the sensor output change rate after the seconddetermination time.

In the PM sensor of the present disclosure, when the amount of PMdepositing between the electrodes as the sensor element becomes equal toor larger than a predetermined amount, electrical continuity may beestablished between the electrodes by deposition of PM. The depositionamount of PM between the electrodes when electrical continuity isestablished between the electrodes of the PM sensor by deposition of PMis referred to as “effective deposition amount of PM”. When thedeposition amount of PM between the electrodes becomes equal to orlarger than the effective deposition amount of PM, the PM sensor maygenerate an output value corresponding to the deposition amount of PMbetween the electrodes. In the PM sensor configured to generate anoutput value that reflects the value of electric current flowing betweenthe electrodes, the output value of the PM sensor may increase with anincrease in deposition amount of PM between the electrodes. In the PMsensor configured to generate an output value that reflects theresistance value between the electrodes, on the other hand, the outputvalue of the PM sensor may decrease with an increase in depositionamount of PM between the electrodes. The PM sensor of the presentdisclosure may have either of these output characteristics, as long asthe PM sensor is configured to output a signal corresponding to thedeposition amount of PM between the electrodes. The larger depositionamount of PM between the electrodes may provide the higher rate ofdecrease in electric resistance between the electrodes relative to theincrease of the deposition amount of PM and provides the higher rate ofincrease in electric current flowing between the electrodes. Whether thePM sensor is configured to generate the output value that reflects theresistance value between the electrodes or is configured to generate theoutput value that reflects the value of electric current flowing betweenthe electrodes, the larger deposition amount of PM between theelectrodes may provide the higher variation in output value of the PMsensor relative to the increase of the deposition amount of PM.

When the deposition amount of PM between the electrodes reaches theeffective deposition amount of PM and is then gradually increased bycontinuation of trapping PM, the output value of the PM sensor may begradually varied with the increase in deposition amount of PM. Whenextraneous substances as well as PM is trapped between the electrodes ofthe PM sensor, electrical continuity may be established between theelectrodes by the trapped extraneous substances to abruptly decrease theresistance value between the electrodes. In this case, the output valueof the PM sensor may be drastically varied, compared with the case wherethe output value of the PM sensor is varied with a gradual increase indeposition amount of PM between the electrodes by trapping PM betweenthe electrodes. In other words, when the output value of the PM sensoris abruptly varied, there may be a high possibility that extraneoussubstances are trapped between the electrodes of the PM sensor.

In the abnormality diagnosis apparatus of the above aspect, the monitorunit may work to continuously monitor the output signal of the PM sensorafter the PM deposition restart time. The PM deposition restart timedenotes a time when deposition of PM between the electrodes of the PMsensor is restarted after completion of the sensor recovery process bythe controller.

The estimated value of the deposition amount of PM between theelectrodes of the PM sensor on the assumption that the filter is in thepredetermined reference state may be specified as the referencedeposition amount of PM. The different state of the filter may providethe different flow rate of PM from the filter. A change in flow rate ofPM from the filter may lead to a change in amount of PM trapped betweenthe electrodes of the PM sensor and may result in changing thedeposition amount of PM between the electrodes. The reference state maydenote a state of the filter assumed for estimation of the referencedeposition amount of PM.

The variation in output value of the PM sensor per unit increase of thereference deposition amount of PM or the variation in output value ofthe PM sensor per unit time may be specified as the sensor output changerate. When the output value of the PM sensor is drastically varied bytrapping extraneous substances between the electrodes of the PM sensoras described above, this can lead to an increase in sensor output changerate. The abnormality diagnosis apparatus of the above aspectaccordingly may compare the sensor output change rate after the PMdeposition restart time but before the first determination time with thedetermination change rate to determine whether the filter diagnosisprocess by the controller is to be performed or not.

The determination change rate can be used as a threshold value fordistinguishing whether the variation in output value of the PM sensor isto be attributed to the gradual increase in deposition amount of PMbetween the electrodes or is to be attributed to the extraneoussubstances trapped between the electrodes. Even in the case where theoutput value of the PM sensor is gradually varied by the gradualincrease in deposition amount of PM between the electrodes, the sensoroutput change rate may not be consistently constant. The largerdeposition amount of PM between the electrodes may provide the higherrate of decrease in resistance value between the electrodes relative tothe increase in deposition amount of PM. The larger deposition amount ofPM between the electrodes can provide the higher rate of increase inelectric current flowing between the electrodes relative to the increasein deposition amount of PM. Accordingly, the larger deposition amount ofPM between the electrodes may provide the higher variation in outputvalue of the PM sensor relative to the increase in deposition amount ofPM. After the PM deposition restart time, the deposition amount of PMbetween the electrodes may be gradually increased by continuouslytrapping PM between the electrodes of the PM sensor. Even whensubstantially no extraneous substances are trapped between theelectrodes, the sensor output change rate may be thus graduallyincreased with an increase in deposition amount of PM between theelectrodes.

In the state with a large amount of PM depositing between the electrodesof the PM sensor, even when substantially no extraneous substances aretrapped between the electrodes, the sensor output change rate in thecase where the output value of the PM sensor is varied by increasing thedeposition amount of PM between the electrodes may exceed thedetermination change rate. In this case, the filter diagnosis process bythe controller can be determined to be not performed, despite thatsubstantially no extraneous substances are trapped between theelectrodes of the PM sensor. After the PM deposition restart time, thedeposition amount of PM between the electrodes of the PM sensorgenerally increases with time, i.e, at the time closer to the firstdetermination time.

In the abnormality diagnosis apparatus of the above aspect, when thesensor output change rate becomes higher than the predetermineddetermination change rate in the time period from the PM depositionrestart time to the predetermined second determination time prior to thefirst determination time, the filter diagnosis process by the controllermay not be performed. In the time period from the PM deposition restarttime to the second determination time, the deposition amount of PMbetween the electrodes of the PM sensor is expected to be less than thedeposition amount of PM after the second determination time. In thistime period, it can thus be distinguishable whether the variation inoutput value of the PM sensor is to be attributed to the gradualincrease in deposition amount of PM between the electrodes or is to beattributed to the extraneous substances trapped between the electrodes,with high accuracy based on the sensor output change rate. In otherwords, when the sensor output change rate becomes higher than thepredetermined determination change rate in this time period, it may bedeterminable that there is a high possibility that extraneous substancesare trapped between the electrodes. In this case, the filter diagnosisprocess can be determined to be not performed. This can reduce wrongdiagnosis of an abnormality of the filter, due to the extraneoussubstances trapped between the electrodes of the PM sensor, despite thatthe filter is actually normal. This may improve the accuracy ofabnormality diagnosis of the filter using the output value of the PMsensor.

When the sensor output change rate does not become higher than thepredetermined determination change rate in the time period from the PMdeposition restart time to the second determination time, on the otherhand, there may be a high possibility that substantially no extraneoussubstances are trapped between the electrodes. It is also expected thatthe deposition amount of PM between the electrodes of the PM sensor isrelatively large after the second determination time. After the seconddetermination time, it accordingly may be difficult to distinguishwhether the variation in output value of the PM sensor is to beattributed to the gradual increase in deposition amount of PM betweenthe electrodes or is to be attributed to the extraneous substancestrapped between the electrodes, with high accuracy based on the sensoroutput change rate. When the sensor output change rate does not becomehigher than the determination change rate in the time period from the PMdeposition restart time to the second determination time, the filterdiagnosis process by the controller may be performed, irrespective ofthe sensor output change rate after the second determination time. Thiscan suppress execution of abnormality diagnosis of the filter from beingunnecessarily prohibited. This may result in suppressing reduction ofthe execution frequency of abnormality diagnosis of the filter beyondnecessity.

In the abnormality diagnosis apparatus of the above aspect, the firstdetermination time may be a time when the reference deposition amount ofPM reaches a predetermined first determination PM deposition amount. Thesecond determination time may be a time when the reference depositionamount of PM reaches a predetermined second determination PM depositionamount. In this aspect, the second determination PM deposition amountmay be less than the first determination PM deposition amount.

In the abnormality diagnosis apparatus of the above aspect, the firstdetermination time may be a time when the reference deposition amount ofPM reaches a predetermined first determination PM deposition amount. Anoutput value of the PM sensor when the deposition amount of PM betweenthe electrodes is a predetermined second determination PM depositionamount that is less than the first determination PM deposition amountmay be specified as a determination output value. When the output valueof the PM sensor reaches the determination output value in a time periodfrom the PM deposition restart time to the first determination time, atime when the output value of the PM sensor reaches the determinationoutput value may be specified as the second determination time.

The abnormality diagnosis apparatus for the filter according to theabove aspect of the invention may further include a differentialpressure sensor that is configured to output a signal corresponding to adifference in exhaust pressure between upstream and downstream of thefilter. And, the controller may further perform a filter recoveryprocess, the filter recovery process being a process of removing PMdepositing on the particulate filter. In this aspect, a state of thefilter may be estimated based on an output value of the differentialpressure sensor at a time of completion of the filter recovery processperformed prior to execution of the sensor recovery process. Thereference deposition amount of PM may be estimated on the assumptionthat the filter is in the estimated state specified as the referencestate. This allows for estimation of the reference deposition amount ofPM on the assumption that the state of the filter is close to the actualstate to some extent. In this aspect, the sensor output change rate maybe a variation in output value of the PM sensor monitored by the monitorunit per unit increase of the reference deposition amount of PM. Thiscan enable the variation in output value of the PM sensor attributed tothe gradual increase of the deposition amount of PM between theelectrodes to be distinguished from the variation in output value of thePM sensor attributed to the extraneous substances trapped between theelectrodes, with the higher accuracy based on the sensor output changerate.

The above aspects of the present disclosure can improve the accuracy ofabnormality diagnosis of the filter using the output value of the PMsensor provided downstream of the filter in the exhaust passage. Thiscan also suppress reduction of the execution frequency of filterabnormality diagnosis beyond necessity. Further features of the presentdisclosure will become apparent from the following description ofexemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a first diagram illustrating the schematic configuration of aninternal combustion engine and its intake and exhaust system accordingto embodiments of the present disclosure;

FIG. 2 is a diagram schematically illustrating the configuration of a PMsensor according to embodiments of the present disclosure;

FIG. 3 is a diagram illustrating a relationship between the depositionamount of PM between electrodes of the PM sensor and the output value ofthe PM sensor according to embodiments of the present disclosure;

FIG. 4 is a diagram showing a variation in output value of the PM sensorafter a voltage applying time according to embodiments of the presentdisclosure;

FIG. 5 is diagrams illustrating variations in output value of the PMsensor and variations in sensor output change rate after the voltageapplying time according to an embodiment of the present disclosurereferred to as Embodiment 1;

FIG. 6 is a flowchart showing a flow of filter abnormality diagnosisaccording to Embodiment 1;

FIG. 7 is a flowchart showing a flow of determination process todetermine whether execution of a filter diagnosis process is to beprohibited or not according to embodiments of the present disclosure;

FIG. 8 is diagrams illustrating variations in output value of the PMsensor and variations in sensor output change rate after the voltageapplying time according to an embodiments of the present disclosurereferred to as Embodiment 2;

FIG. 9 is a flowchart showing a flow of filter abnormality diagnosisaccording to Embodiment 2; and

FIG. 10 is a second diagram illustrating the schematic configuration ofan internal combustion engine and its intake and exhaust systemaccording to embodiments of the present disclosure.

DETAILED DESCRIPTION

The following describes embodiments of the present disclosure withreference to the drawings. The dimensions, the materials, the shapes,the positional relationships and the like of the respective componentsdescribed in the following embodiments are only for the purpose ofillustration and not intended at all to limit the scope of the presentdisclosure to such specific descriptions.

FIG. 1 is a diagram illustrating the schematic configuration of aninternal combustion engine 1 and its intake and exhaust system accordingto an embodiment of the present disclosure, referred to as Embodiment 1.

The internal combustion engine 1 shown in FIG. 1 is a compressionignition internal combustion engine, such as a diesel engine, usinglight oil as fuel. The internal combustion engine 1 may alternatively bea spark ignition internal combustion engine using gasoline or the likeas fuel.

The internal combustion engine 1 includes a fuel injection valve 3 thatis configured to inject the fuel into a cylinder 2. In the case of theinternal combustion engine 1 provided as the spark ignition internalcombustion engine, the fuel injection valve 3 may be configured toinject the fuel into an intake port.

The internal combustion engine 1 is connected with an intake passage 4.The intake passage 4 is provided with an air flow meter 40 and an intakethrottle valve 41. The air flow meter 40 is configured to output anelectric signal corresponding to the amount, such as mass, of the intakeair flowing in the intake passage 4. The intake throttle valve 41 isplaced downstream of the air flow meter 40 in the intake passage 4. Theintake throttle valve 41 is configured to change the passagecross-sectional area of the intake passage 4 and thereby regulate theamount of the air taken into the internal combustion engine 1.

The internal combustion engine 1 is also connected with an exhaustpassage 5. The exhaust passage 5 is provided with an oxidation catalyst50 and a particulate filter (hereinafter simply referred to as “filter”)51. The filter 51 is placed downstream of the oxidation catalyst 50 inthe exhaust passage 5. The filter 51 is a wall-flow filter that is madeof a porous base material and is configured to trap PM included in theexhaust gas.

A fuel addition valve 52 is placed upstream of the oxidation catalyst 50in the exhaust passage 5. The fuel addition valve 52 is configured toadd the fuel to the exhaust gas flowing in the exhaust passage 5. Theexhaust passage 5 is also provided with a temperature sensor 54 and a PMsensor 55 that are located downstream of the filter 51. The temperaturesensor 54 is configured to output an electric signal corresponding tothe temperature of the exhaust gas. The PM sensor 55 is configured tooutput an electric signal relating to the amount of PM flowing out ofthe filter 51.

The schematic configuration of the PM sensor 55 is described withreference to FIG. 2. FIG. 2 is a diagram illustrating the schematicconfiguration of the PM sensor 55. The PM sensor 55 is anelectrode-based PM sensor. The PM sensor 55 includes only one set ofelectrodes in the illustrated example of FIG. 2 but may include multiplesets of electrodes.

The PM sensor 55 includes a sensor element 553, an ammeter 554, a heater555 and a cover 556. The sensor element 553 is configured by a pair ofelectrodes 551 and 552 that are placed away from each other on a surfaceof a plate-like insulator 550. The ammeter 554 is configured to measurethe electric current flowing between the electrodes 551 and 552. Theheater 555 is an electric heater placed on a rear face of the insulator550. The cover 556 is provided to cover the sensor element 553. Thecover 556 has a plurality of through holes 557 formed therein. Electricpower is supplied from an external power source 60 to the electrodes 551and 552 and the heater 555 of the PM sensor 55. The PM sensor 55generates an output value that reflects the value of electric currentmeasured by the ammeter 554. The output value of the PM sensor 55 isinput into a monitor unit 101 of an ECU 10. According to thisembodiment, the output value of the PM sensor 55 may thus becontinuously monitored by the monitor unit 101 of the ECU 10. In thecase where the PM sensor 55 is provided with a sensor control unit (SCU)of controlling the PM sensor 55, the SCU may include a monitor unitconfigured to continuously monitor the output value of the PM sensor 55.The SCU may be programmed to perform functions and processes disclosedherein.

In the state that the PM sensor 55 having the above configuration ismounted to the exhaust passage 5, part of the exhaust gas flowing in theexhaust passage 5 flows into the cover 556 via the through holes 557. PMincluded in the exhaust gas flowing into the cover 556 is trappedbetween the electrodes 551 and 552. Trapping PM between the electrodes551 and 552 is accelerated by applying a voltage to the electrodes 551and 552.

The following describes the relationship between the deposition amountof PM between the electrodes 551 and 552 and the output value of the PMsensor 55 with reference to FIG. 3. The abscissa of FIG. 3 shows thedeposition amount of PM between the electrodes 551 and 552, and theordinate of FIG. 3 shows the output value of the PM sensor 55. TrappingPM between the electrodes 551 and 552 results in gradually increasingthe deposition amount of PM between the electrodes 551 and 552. In thestate that a voltage is applied between the electrodes 551 and 552,deposition of a predetermined amount of PM between the electrodes 551and 552 such as to connect from one electrode 551 to the other electrode552 provides electrical continuity between the electrodes 551 and 552,due to the electrical conductivity of PM. When the deposition amount ofPM between the electrodes 551 and 552 is less than the predeterminedamount, however, there is no electrical continuity between theelectrodes 551 and 552. The deposition amount of PM that provideselectrical continuity between the electrodes 551 and 552 is hereinafterreferred to as “effective deposition amount of PM”.

As shown in FIG. 3, there is no electrical continuity between theelectrodes 551 and 552 until the deposition amount of PM between theelectrodes 551 and 552 reaches the effective deposition amount of PM, sothat the output value of the PM sensor 55 is equal to zero. When thedeposition amount of PM between the electrodes 551 and 552 reaches theeffective deposition amount of PM, the output value of the PM sensor 55becomes greater than zero. After the deposition amount of PM between theelectrodes 551 and 552 reaches the effective deposition amount of PM,the electric resistance between the electrodes 551 and 552 decreaseswith an increase in deposition amount of PM between the electrodes 551and 552. This results in increasing the electric current flowing betweenthe electrodes 551 and 552. The output value of the PM sensor 55accordingly increases with an increase in deposition amount of PMbetween the electrodes 551 and 552. In the description below, the timewhen the output value of the PM sensor 55 starts increasing from zero iscalled “output starting time”. The larger deposition amount of PMbetween the electrodes 551 and 552 provides the higher rate of decreasein electric resistance between the electrodes 551 and 552 relative tothe increase in deposition amount of PM and thereby provides the higherrate of increase in electric current flowing between the electrodes 551and 552. The larger deposition amount of PM between the electrodes 551and 552 accordingly provides the higher rate of increase in output valueof the PM sensor 55 relative to the increase in deposition amount of PM.

Referring back to FIG. 1, the internal combustion engine 1 is providedwith the electronic control unit (ECU) 10. The ECU 10 is a unit servingto control, for example, the operating conditions of the internalcombustion engine 1. The ECU 10 is electrically connected with varioussensors including an accelerator positions sensor 7 and a crank positionsensor 8, in addition to the air flow meter 40, the temperature sensor54 and the PM sensor 55 described above. The accelerator position sensor7 is provided as a sensor that outputs an electric signal related to theoperation amount (e.g., accelerator position) of an accelerator pedal(not shown). The crank position sensor 8 is provided as a sensor thatoutputs an electric signal related to the rotational position of anoutput shaft (e.g., the crankshaft) of the internal combustion engine 1.The output signals of these sensors are input into the ECU 10. The ECU10 is also electrically connected with various devices such as the fuelinjection valve 3, the intake air throttle valve 41 and the fueladdition valve 52 described above. The ECU 10 controls the above variousdevices, based on the output signals from the above various sensors, andmay be programmed to perform the functions and processes disclosedherein. For example, the ECU 10 performs a filter recovery process toremove PM depositing on the filter 51 by addition of the fuel by thefuel addition valve 52. The filter recovery process increases thetemperature of the filter 51 with the heat generated by oxidation of thefuel added by the fuel addition valve 52 in the oxidation catalyst 50.This results in oxidizing and removing PM depositing on the filter 51.

A failure such as breakage or erosion may occur in the filter 51 due to,for example, a temperature rise during the above filter recoveryprocess. Such a failure occurring in the filter 51 or an abnormality offilter, for example, detachment of the filter 51 from the exhaustpassage 5, increases PM released to the atmosphere. Filter abnormalitydiagnosis may therefore be performed to determine whether the filter hasany abnormality, based on the output value of the PM sensor 55. Thefollowing describes a procedure of filter abnormality diagnosisaccording to embodiments of the present disclosure.

The procedure of filter abnormality diagnosis may first perform thesensor recovery process, in order to remove PM depositing between theelectrodes 551 and 552 of the PM sensor 55. More specifically, electricpower is supplied from the power source 60 to the heater 555, and theheater 555 then heats the sensor element 553. This results in oxidizingand removing PM depositing between the electrodes 551 and 552. In thesensor recovery process, the temperature of the sensor element 553 iscontrolled to a temperature that allows for oxidation of PM by adjustingthe supply amount of electric power to the heater 555.

After performing the sensor recovery process to remove PM depositingbetween the electrodes 551 and 552, the procedure subsequently startsapplying a voltage from the power source 60 to the electrodes 551 and552. In the description below, the time when applying a voltage to theelectrodes 551 and 552 is started is called “voltage applying time.” Theelectrodes 551 and 552 have high temperature for some time aftercompletion of the sensor recovery process. A cooling time period forcooling down the electrodes 551 and 552 may thus be provided betweencompletion of the sensor recovery process and the voltage applying time.

As described above, applying a voltage to the electrodes 551 and 552accelerates trapping PM between the electrodes 551 and 552. According tothis embodiment, the voltage applying time thus corresponds to the PMdeposition restart time of the invention. According to this embodiment,applying a voltage to the electrodes 551 and 552 may be started duringthe sensor recovery process. In this case, the time when the sensorrecovery process is completed (i.e., the time when power supply to theheater 555 is stopped) may be specified as the PM deposition restarttime of the invention. The time when a predetermined time period fordetermining that the temperature of the electrodes 551 and 552 of the PMsensor 55 is decreased to such a degree that does not oxidize thetrapped PM has elapsed since completion of the sensor recovery processmay be specified as the PM deposition restart time of the invention.

The following describes a behavior of the output value of the PM sensor55 after a start of applying a voltage to the electrodes 551 and 552.FIG. 4 is a diagram showing a variation in output value of the PM sensor55 after the voltage applying time. The abscissa of FIG. 4 shows areference deposition amount of PM between the electrodes 551 and 552 ofthe PM sensor 55 after the voltage applying time, and the ordinate ofFIG. 4 shows the output value of the PM sensor 55.

According to embodiments of the present disclosure, the referencedeposition amount of PM is an estimated value on the assumption that thefilter 51 is in a reference failure state. The reference failure statedenotes a state of a slightest failure among the states that the filter51 is to be determined as abnormal by filter abnormality diagnosis. Evenin the state that the filter 51 deteriorates to some extent, when thedeteriorating state is better than the reference failure state, thefilter 51 is determined as normal by filter abnormality diagnosis. Thereference deposition amount of PM is calculated by estimating the amountof PM trapped between the electrodes 551 and 552 of the PM sensor 55(hereinafter simply referred to as “trapped amount of PM”) on theassumption that the filter 51 is in the reference failure state andintegrating the estimated value of the trapped amount of PM. Even in thecase where the state of the filter 51 itself is unchanged, the amount ofPM flowing out of the filter 51 is varied according to the operatingconditions of the internal combustion engine 1 (for example, the amountof fuel injection from the fuel injection valve 3 and the flow rate ofthe exhaust gas) and the deposition amount of PM on the filter 51. Theratio of the amount of PM trapped between the electrodes 551 and 552 ofthe PM sensor 55 to the amount of PM included in the exhaust gas is alsovaried according to the flow rate of the exhaust gas. Estimation of thetrapped amount of PM on the assumption that the filter 51 is in thereference failure state should accordingly take into account theoperating conditions of the internal combustion engine 1 and thedeposition amount of PM on the filter 51. Any known method may beemployed to concretely calculate the reference deposition amount of PM.

In the graph of FIG. 4, a curve L1 shows a variation in output value ofthe PM sensor 55 in the normal state of the filter 51, and a curve L2shows a variation in output value of the PM sensor 55 in the failedstate of the filter 51. A variation in output value of the PM sensor 55in the state that the filter 51 is detached from the exhaust passage 5shows a similar tendency to the variation in the failed state of thefilter 51 relative to the variation in the normal state of the filter51. In the graph of FIG. 4, Qs1 represents a reference deposition amountof PM at the output starting time in the normal state of the filter 51,and Qs2 represents a reference deposition amount of PM at the outputstarting time in the failed state of the filter 51. The behavior of theoutput value of the PM sensor 55 as shown in FIG. 4 may be monitored bythe monitor unit 101 of the ECU 10.

A failure of the filter 51 decreases the PM trapping efficiency of thefilter 51. This results in increasing the amount of PM flowing out ofthe filter 51 per unit time (flow-out amount of PM). This accordinglyincreases the amount of PM that reaches the PM sensor 55 and is trappedbetween the electrodes 551 and 552. This leads to a higher increase rateof the deposition amount of PM between the electrodes 551 and 552. As aresult, in the failed state of the filter 51, the deposition amount ofPM between the electrodes 551 and 552 reaches the effective depositionamount of PM earlier than in the normal state of the filter 51.Accordingly, as shown in FIG. 4, the failed state of the filter 51 has ashorter time period from the voltage applying time to the outputstarting time than the normal state of the filter 51 (Qs2<Qs1). Thefailed state of the filter 51 also has a higher increase rate of thedeposition amount of PM between the electrodes 551 and 552 after theoutput starting time than the normal state of the filter 51. The failedstate of the filter 51 accordingly has a higher rate of change of thesensor output after the output starting time than the normal state ofthe filter 51 as shown in FIG. 4. The rate of change of the sensoroutput denotes an increase rate of the output value of the PM sensor 55per unit increase of the reference deposition amount of PM.

There is a difference in behavior of the output value of the PM sensor55 between the normal state of the filter 51 and the abnormal state ofthe filter 51 as described above. As a result, the abnormal state of thefilter 51 provides a larger output value of the PM sensor 55 afterelapse of a predetermined time period since the voltage applying timethan the normal state of the filter 51. Accordingly, the procedure offilter abnormality diagnosis according to this embodiment reads theoutput value of the PM sensor 55 at a first determination time td1 afterelapse of a predetermined determination time period dtd since thevoltage applying time. When the read output value of the PM sensor 55 isequal to or higher than a predetermined abnormality determination valueSth, it is determined that the PM sensor 55 is abnormal. Thedetermination time period dtd is set as a time duration from the voltageapplying time to the time when the reference deposition amount of PMreaches a predetermined first determination PM deposition amount Qpme1.

There is a likelihood that conductive materials other than PM as theoriginal detection object (e.g., extraneous substances) are trappedbetween the electrodes 551 and 552 of the PM sensor 55. For example,moisture included in the exhaust gas is condensed to produce condensedwater in the exhaust passage 5. The condensed water may enter the PMsensor 55 and may be trapped between the electrodes 551 and 552. In oneexample, the exhaust passage may be provided with a selective reductionNOx catalyst and a urea addition valve. The selective reduction NOxcatalyst denotes a catalyst that uses ammonia as a reducing agent toreduce NOx in the exhaust gas. The urea addition valve is operated toadd urea water for producing ammonia as the reducing agent to theexhaust gas. In a configuration that this urea addition valve is placedupstream of the PM sensor 55 in the exhaust passage 5, urea (e.g., ureadeposit) precipitating from urea water may be trapped between theelectrodes 551 and 552 of the PM sensor 55. Such condensed water andurea deposit are not the original detection object of the PM sensor 55and should thus be regarded as extraneous substances.

According to embodiments of the present disclosure, part of PM includedin the exhaust gas adheres to the wall surface of the exhaust passage 5and various structures provided in the exhaust passage 5 such as thedownstream-side end face of the filter 51 and the oxidation catalyst 50(hereinafter referred to as “exhaust system structures”). The followingphenomenon has also be found: PM once adhering to the wall surface ofthe exhaust passage 5 and the exhaust system structures and then fallingoff from the wall surface and the exhaust system structures (hereinafterreferred to as “fall-off PM”) may reach the PM sensor 55 and may betrapped between the electrodes 551 and 652. The original detectionobject of the PM sensor 55 is ordinary PM that is included in theexhaust gas discharged from the internal combustion engine 1 and reachesthe PM sensor 55 without adhering to the wall surface of the exhaustpassage 5 and the exhaust system structures. In other words, thefall-off PM is not the original detection object of the PM sensor 55.Accordingly, the fall-off PM is also a kind of extraneous substance,like the condensed water and the urea deposit described above.

Even in the normal state of the filter 51, such extraneous substancesare likely to be trapped between the electrodes 551 and 552 of the PMsensor 55. The following describes the effects of such extraneoussubstances trapped between the electrodes 551 and 552 of the PM sensor55 on the output value of the PM sensor 55 with reference to FIG. 5. Theupper graph of FIG. 5 is a diagram illustrating variations in outputvalue of the PM sensor 55 after the voltage applying time. In the uppergraph of FIG. 5, the abscissa shows the reference deposition amount ofPM after the voltage applying time, and the ordinate shows the outputvalue of the PM sensor 55. In the upper graph of FIG. 5, a curve L3shows a variation in output value of the PM sensor 55 in the failedstate of the filter 51. More specifically, the curve L3 shows avariation in output value of the PM sensor 55 in the case where thedeposition amount of PM between the electrodes 551 and 552 is graduallyincreased by trapping the ordinary PM between the electrodes 551 and 552of the PM sensor 55. In the upper graph of FIG. 5, a curve L4, on theother hand, shows a variation in output value of the PM sensor 55 in thenormal state of the filter 51. More specifically, the curve L4 shows avariation in output value of the PM sensor 55 in the case where theextraneous substances as well as the ordinary PM is trapped between theelectrodes 551 and 552 of the PM sensor 55. The lower graph of FIG. 5 isa diagram illustrating variations in sensor output change rate of the PMsensor 55 after the voltage applying time. In the lower graph of FIG. 5,the abscissa shows the reference deposition amount of PM after thevoltage applying time, and the ordinate shows the sensor output changerate. In the lower graph of FIG. 5, a curve L5 shows a variation insensor output change rate corresponding to the variation in output valueof the PM sensor 55 shown by the curve L3 in the upper graph of FIG. 5.In the lower graph of FIG. 5, a curve L6 shows a variation in sensoroutput change rate corresponding to the variation in output value of thePM sensor 56 shown by the curve L4 in the upper graph of FIG. 5.

When the extraneous substances are trapped between the electrodes 551and 552 of the PM sensor 55, the trapped extraneous substances establishelectrical continuity between the electrodes 551 and 552 and therebydecrease the resistance value between the electrodes 551 and 552.Accordingly, in the case where the extraneous substances are trappedbetween the electrodes 551 and 552 before the deposition amount of PMbetween the electrodes 551 and 552 of the PM sensor 55 reaches theeffective deposition amount of PM, the output value of the PM sensor 55starts increasing from zero at this time. In this case, as shown by thecurve L4 in the upper graph of FIG. 5, the output value of the PM sensor55 abruptly increases at the output starting time. Compared with thecase where the deposition amount of PM between the electrodes 551 and552 is gradually increased to exceed the effective deposition amount ofPM, the output value of the PM sensor 55 drastically increasesimmediately after the output starting time in this case. As a result,the output value of the PM sensor 55 is likely to become larger than theabnormality determination value Sth at the first determination time td1,despite that the filter 51 is actually normal.

Accordingly, in the case where the extraneous substances are trappedbetween the electrodes 551 and 552 of the PM sensor 55 in a time periodfrom the voltage applying time to the first determination time td1, inthe process of filter abnormality detection using the output value ofthe PM sensor 55 at the first determination time td1, there is alikelihood of wrong diagnosis that the filter 51 is abnormal, despitethat the filter 51 is actually normal.

According to embodiments of the present disclosure, the output signal ofthe PM sensor 55 after the voltage applying time is continuouslymonitored by the monitor unit 101 of the ECU 10. The referencedeposition amount of PM after the voltage applying time is alsocontinuously estimated by the ECU 10. The procedure of this embodimentcompares the sensor output change rate after the voltage applying timecalculated from the output value of the PM sensor and the estimatedreference deposition amount of PM with a predetermined determinationchange rate and determines whether filter abnormality diagnosis usingthe output value of the PM sensor 55 is to be performed or not. Thedetermination change rate herein is a threshold value for distinguishingwhether the increase in output value of the PM sensor 55 is to beattributed to the gradual increase in deposition amount of PM betweenthe electrodes 551 and 552 or is to be attributed to the extraneoussubstances trapped between the electrodes 551 and 552. In the lowergraph of FIG. 5, this determination change rate is expressed as Rth.

As described above, in the case where the extraneous substances aretrapped between the electrodes 551 and 552 of the PM sensor 55, theoutput value of the PM sensor 55 abruptly increases, compared with thecase where the deposition amount of PM between the electrodes 551 and552 is gradually increased by trapping PM between the electrodes 551 and552. Accordingly, as shown by the curve L6 in the lower graph of FIG. 5,the sensor output change rate at the time when the extraneous substancesare trapped (i.e., sensor output change rate at the output starting timein the curve L6) in the case where the extraneous substances are trappedbecomes higher than the sensor output change rate in the case where thedeposition amount of PM between the electrodes 551 and 552 is graduallyincreased. In other words, when the sensor output change rate isdrastically increased after the voltage applying time, it isdeterminable that there is a high possibility that the extraneoussubstances are trapped between the electrodes 551 and 552 of the PMsensor 55. In this case, the procedure accordingly determines that thefilter diagnosis process of diagnosing an abnormality of the filter 51based on the output value of the PM sensor 55 at the first determinationtime td1 is not to be performed.

Even in the case where the output value of the PM sensor 55 is graduallyvaried by the gradual increase in deposition amount of PM between theelectrodes 551 and 552, the sensor output change rate is notconsistently constant. As described above, the larger deposition amountof PM between the electrodes 551 and 552 provides the higher rate ofincrease in output value of the PM sensor 55 relative to the increase indeposition amount of PM. In general, the amount of PM actuallydepositing between the electrodes 551 and 552 increases with an increasein reference deposition amount of PM. Accordingly, as shown by the curveL5 in the lower graph of FIG. 5, the sensor output change rate graduallyincreases with an increase in reference deposition amount of PM evenwhen substantially no extraneous substances are trapped between theelectrodes 551 and 552.

At a time close to the first determination time td1, there isaccordingly a large deposition amount of PM between the electrodes 551.The sensor output change rate in the case where the output value of thePM sensor 55 is varied by increasing the deposition amount of PM betweenthe electrodes 551 and 552 is thus likely to become comparable to thesensor output change rate in the case where the output value of the PMsensor 55 is varied by trapping the extraneous substances between theelectrodes 551 and 552. Accordingly, as shown by the curve L5 in thelower graph of FIG. 5, at the time close to the first determination timetd1, even when substantially no extraneous substances are trappedbetween the electrodes 551 and 552, the sensor output change rate in thecase where the output value of the PM sensor 55 is varied by increasingthe deposition amount of PM between the electrodes 551 and 552 is likelyto exceed the determination change rate Rth. In this case, it isdetermined that the filter diagnosis process is not to be performed,despite that substantially no extraneous substances are trapped betweenthe electrodes 551 and 552 of the PM sensor 55. As a result, as shown bythe curve L3 in the upper graph of FIG. 5, a failure of the filter 51 isnot detectable despite a variation in output value of the PM sensor 55that indicates a failure of the filter 51.

According to embodiments of the present disclosure, a time when thereference deposition amount of PM reaches a predetermined seconddetermination PM deposition amount Qpme2 that is less than the firstdetermination PM deposition amount Qpme1 after the voltage applying timeis specified as a second determination time td2, as shown in FIG. 5. Theprocedure determines whether the filter diagnosis process is to beperformed or not, based on the sensor output change rate in a timeperiod from the voltage applying time to the second determination timetd2.

It is expected that the deposition amount of PM between the electrodes551 and 552 of the PM sensor 55 in the time period from the voltageapplying time to the second determination time td2 is less than thedeposition amount of PM after the second determination time td2. In thistime period, it is thus distinguishable whether the increase in outputvalue of the PM sensor 55 is to be attributed to the gradual increase indeposition amount of PM between the electrodes 551 and 552 or is to beattributed to the extraneous substances trapped between the electrodes551 and 552, with high accuracy based on the sensor output change rate.In other words, when the sensor output change rate becomes higher thanthe determination change rate Rth in this time period, it isdeterminable that there is a high possibility that extraneous substancesare trapped between the electrodes 551 and 552. Accordingly, in the casewhere the sensor output change rate becomes higher than thedetermination change rate Rth in the time period from the voltageapplying time to the second determination time td2, the procedure of theembodiment determines that the filter diagnosis process is not to beperformed. This reduces wrong diagnosis of an abnormality of the filter51 due to the extraneous substances trapped between the electrodes 551and 552 of the PM sensor 55, despite that the filter 51 is actuallynormal. This results in improving the accuracy of abnormality diagnosisof the filter 51 using the output value of the PM sensor 55.

In the case where the sensor output change rate does not become higherthan the determination change rate Rth in the time period from thevoltage applying time to the second determination time td2, on the otherhand, it is determinable that there is a high possibility thatsubstantially no extraneous substances are trapped between theelectrodes 551 and 552. It is also expected that the deposition amountof PM between the electrodes 551 and 552 of the PM sensor 55 isrelatively large after the second determination time td2. After thesecond determination time td2, it is accordingly difficult todistinguish whether the increase in output value of the PM sensor 55 isto be attributed to the gradual increase in deposition amount of PMbetween the electrodes 551 and 552 or is to be attributed to theextraneous substances trapped between the electrodes 551 and 552, withhigh accuracy based on the sensor output change rate. When the sensoroutput change rate does not become higher than the determination changerate Rth in the time period from the voltage applying time to the seconddetermination time td2, the procedure of this embodiment performs thefilter diagnosis process based on the output value of the PM sensor 55at the first determination time td1, irrespective of the sensor outputchange rate after the second determination time td2. This suppressesexecution of abnormality diagnosis of the filter 51 from beingunnecessarily prohibited. This results in suppressing reduction of theexecution frequency of abnormality diagnosis of the filter 51 beyondnecessity.

The following describes a flow of filter abnormality diagnosis of theembodiment with reference to FIG. 6. FIG. 6 is a flowchart showing theflow of filter abnormality diagnosis of the embodiment. This flow isperformed by the ECU 10 on satisfaction of predetermined filterdiagnosis preliminary conditions. The filter diagnosis preliminaryconditions herein are conditions to perform a sensor recovery processprior to the filter diagnosis process. The filter diagnosis preliminaryconditions are set to necessarily and sufficiently ensure the executionfrequency of the filter diagnosis process. The filter diagnosispreliminary conditions may be, for example, that the internal combustionengine 1 is in the state of steady operation and that a predeterminedtime period has elapsed since previous execution of the filter diagnosisprocess or that a predefined time period has elapsed since a currentstart of operation of the internal combustion engine 1. In anapplication that the PM sensor 55 is provided with an SCU, this flow maybe performed by the SCU.

This flow first determines whether the PM sensor 55 is normal at S101.According to this embodiment, a flow of failure diagnosis of the PMsensor 55 is performed as a separate routine from this flow, and theresult of failure diagnosis is stored in the ECU 10. At S101, the flowreads the result of failure diagnosis of the PM sensor 55 stored in theECU 10. When the result of diagnosis indicating a failure of the PMsensor 55 is stored in the ECU 10, a negative answer is provided atS101. In this case, the flow is terminated. When the result of diagnosisindicating a failure of the PM sensor 55 is not stored in the ECU 10, onthe other hand, an affirmative answer is provided at S101. In this case,the flow proceeds to the process of S102. Any of known techniques may beemployed for failure diagnosis of the PM sensor 55.

At S102, the sensor recovery process is performed. The sensor recoveryprocess supplies electric power from the power source 60 to the heater555 and controls the temperature of the sensor element 553 to atemperature that allows for oxidation of PM. At S102, the supply ofelectric power to the heater 555 is continued until elapse of apredetermined sensor recovery time since the start of power supply. Thesensor recovery time may be a fixed value determined in advance byexperiment or the like as a sufficient time duration for removal of PMdepositing between the electrodes 551 and 552 of the PM sensor 55. Thesensor recovery time may be set, based on an estimated deposition amountof PM between the electrodes 551 and 552 at the start of the sensorrecovery process. When the sensor recovery time has elapsed since thestart of supply of electric power to the heater 555, the supply ofelectric power from the power source 60 to the heater 555 is stopped, sothat the sensor recovery process is completed. The deposition amount ofPM between the electrodes 551 and 552 of the PM sensor 55 isapproximately zero at the time when the sensor recovery process iscompleted.

The flow subsequently performs the process of S103. At S103, the flowstarts application of a voltage to the electrodes 551 and 552 of the PMsensor 55. This accelerates trapping PM between the electrodes 551 and552. According to embodiments of the present disclosure, at the start ofapplication of a voltage to the electrodes 551 and 552 of the PM sensor55, the monitor unit 101 of the ECU 10 may start monitoring the outputvalue of the PM sensor 55. A cooling time period for cooling down theelectrodes 551 and 552 may be provided between completion of the sensorrecovery process and start of applying a voltage to the electrodes 551and 552 of the PM sensor 55, as described above. The application starttime of a voltage to the electrodes 551 and 552 of the PM sensor 55 maynot be necessarily simultaneous with the monitor start time of theoutput value of the PM sensor 55 by the monitor unit 101 of the ECU 10.

At S104, the flow subsequently performs a determination process ofdetermining whether execution of the filter diagnosis process is to beprohibited. The following describes a flow of the determination processof the embodiment with reference to FIG. 7. FIG. 7 is a flowchartshowing the flow of the determination process of the embodiment. Thisflow is repeatedly performed by the ECU 10 after start of application ofa voltage to the electrodes 551 and 552 of the PM sensor 55. In theapplication that the PM sensor 55 is provided with an SCU, this flow mayalso be performed by the SCU.

At S201, the flow calculates a reference deposition amount of PM Qpme atthe current moment. The reference deposition amount of PM Qpme iscalculated, based on the operating conditions of the internal combustionengine 1 and the deposition amount of PM on the filter 51 on theassumption that the filter 51 is in the reference failure state. Thedeposition amount of PM on the filter 51 on the assumption that thefilter 51 is in the reference failure state may be calculated byestimating the amount of PM trapped by the filter 51 on the assumptionthat the filter 51 is in the reference failure state and the removalamount of PM oxidized by increasing the temperature of the exhaust gasto be removed from the filter 51 and integrating these estimated values.

At S202, the flow subsequently obtains an output value Sout of the PMsensor 55 at the current moment. At S203, the flow then calculates asensor output change rate Rsout of the PM sensor 55. In thiscalculation, the reference deposition amount of PM calculated at S201 ina previous cycle of this flow is defined as a first determination PMdeposition amount Qpme1, and the reference deposition amount of PMcalculated at S201 in a current cycle of this flow is defined as asecond determination PM deposition amount Qpme2. The output value of thePM sensor obtained at S202 in the previous cycle of this flow is definedas a first output value Sout1, and the output value of the PM sensorobtained at S202 in the current cycle of this flow is defined as asecond output value Sout2. At S203, the sensor output change rate Rsoutis calculated according to Equation (1) given below:

Rsout=(Sout2−Sout1)/(Qpme2·Qpme1)  (1)

The sensor output change rate Rsout is thus calculated as a ratio of thedifference between the output values of the PM sensor at two differenttimes of execution of this flow to the difference between the referencedeposition amounts of PM at the two different times.

At S204, the flow subsequently determines whether the sensor outputchange rate Rsout calculated at S203 is higher than a determinationchange rate Rth. According to embodiments of the present disclosure, thedetermination change rate Rth may be specified in advance by experimentor the like and is stored in the ECU 10. In the case of an affirmativeanswer at S204, there is a high possibility that extraneous substancesare trapped between the electrodes 551 and 552 of the PM sensor 55. Inthis case, the flow determines that the filter diagnosis process is notto be performed based on the output value of the PM sensor 55 at thefirst determination time td1. Accordingly, in the case of an affirmativeanswer at S204, the flow sets a diagnosis prohibition flag ON at S205.In the case of a negative answer at S204, on the other hand, i.e, in thecase where the sensor output change rate Rsout calculated at S203 isequal to or lower than the determination change rate Rth, there is ahigh possibility that substantially no extraneous substances are trappedbetween the electrodes 551 and 552 at the current moment. In this case,the flow sets the diagnosis prohibition flag OFF at S206.

The description is referred back to the flow of filter abnormalitydiagnosis shown in FIG. 6. This flow performs the process of S105 afterthe process of S104. At S105, the flow determines whether the diagnosisprohibition flag is set ON by the determination process performed atS104. In the case of a negative answer at S105, i.e., when the diagnosisprohibition flag is OFF, the flow proceeds to the process of S106. AtS106, the flow determines whether the current reference depositionamount of PM Qpme is equal to or larger than the predetermined seconddetermination PM deposition amount Qpme2. This determines whether thetime comes to the second determination time td2. In the case of anegative answer at S106, i.e., when the reference deposition amount ofPM Qpme has not yet reached or exceeded the second determination PMdeposition amount Qpme2, the determination process is performed again atS104.

In the case of an affirmative answer at S106, on the other hand, it isdetermined that the reference deposition amount of PM Qpme has reachedor exceeded the second determination PM deposition amount Qpme2 withoutsetting the diagnosis prohibition flag ON in the determination processof S104 after the voltage applying time. In other words, it isdetermined that substantially no extraneous substances are trappedbetween the electrodes 551 and 552 of the PM sensor 55 in a time periodfrom the voltage applying time to the second determination time td2.Accordingly, in this case, the flow determines that the filter diagnosisprocess is to be performed based on the output value of the PM sensor 55at the first determination time td1. At S107, the flow subsequentlydetermines whether the current reference deposition amount of PM Qpme isequal to or larger than the first determination PM deposition amountQpme1. This determines whether the time comes to the first determinationtime td1 after elapse of the determination time period dtd since thevoltage applying time. The process of S107 is repeatedly performed untilan affirmative answer is provided at S107.

In the case of an affirmative answer at S107, the flow subsequentlyperforms the filter diagnosis process at S108. More specifically, thefilter diagnosis process determines whether the output value Sout of thePM sensor 55 is equal to or larger than the abnormality determinationvalue Sth when the reference deposition amount of PM Qpme has reachedthe first determination PM deposition amount Qpme1. In the case of anaffirmative answer at S108, the flow proceeds to S109 to determine thatthe filter 51 is abnormal. In the case of a negative answer at S108, onthe other hand, the flow proceeds to S110 to determine that the filter51 has no abnormality but is normal. After determining that the filter51 is abnormal at S109 or determining that the filter 51 is normal atS110, the flow stops application of a voltage to the electrodes 551 and552 of the PM sensor 55 at S111.

In the case of an affirmative answer at S105, on the other hand, theflow subsequently stops application of a voltage to the electrodes 551and 552 of the PM sensor 55 at S111. This accordingly stops applicationof a voltage to the electrodes 551 and 552 without performing the filterdiagnosis process. In the case of an affirmative answer at S105, it is,however, not necessarily required to stop application of a voltage tothe electrodes 551 and 552 of the PM sensor 55 immediately. Even in thecase of an affirmative answer at S105, a modification may continueapplication of a voltage to the electrodes 551 and 552 until thereference deposition amount of PM Qpme reaches the first determinationPM deposition amount Qpme1. This modification also determines that thefilter diagnosis process is not to be performed based on the outputvalue of the PM sensor 55 at the time when the reference depositionamount of PM Qpme reaches the first determination PM deposition amountQpme1. In the case of an affirmative answer at S105, anothermodification may perform the sensor recovery process to remove theextraneous substances trapped between the electrodes 551 and 552 andsubsequently perform the process of and after S103 again.

The flow of filter abnormality diagnosis described above determines thatthe filter diagnosis process is not to be performed in the case whereextraneous substances are trapped between the electrodes 551 and 552 ofthe PM sensor 55 in the time period from the voltage applying time tothe second determination time td2. In the case where substantially noextraneous substances are trapped between the electrodes 551 and 552 ofthe PM sensor 55 in the time period from the voltage applying time tothe second determination time td2, the flow determines that the filterdiagnosis process is to be performed, irrespective of the sensor outputchange rate after the second determination time td2.

In one embodiment, referred to as Embodiment 2, a filter diagnosisprocess by a procedure similar to that of the previously discussedEmbodiment 1 may be performed. More specifically, the time when thereference deposition amount of PM reaches the predetermined firstdetermination PM deposition amount Qpme1 after the voltage applying timeis defined as the first determination time td1. It is determined thatthe PM sensor 55 is abnormal when the output value of the PM sensor 55at the first determination time td1 is equal to or larger than thepredetermined abnormality determination value Sth. Embodiment 2,however, employs a procedure different from that of Embodiment 1, todetermine whether the filter diagnosis process is to be performed or notmay be utilized.

FIG. 8 is a diagram illustrating variations in output value of the PMsensor 55 and variations in sensor output change rate after the voltageapplying time, like FIG. 5 described above. Curves L3 to L6 in FIG. 8show variations in output value of the PM sensor 55 and variations insensor output change rate, similar to the curves L3 to L6 in FIG. 5.Like FIG. 5, td1 in FIG. 8 indicates a first determination time, and td2in FIG. 8 indicates a second determination time. A determination outputvalue Sout0 represents the output value of the PM sensor 55 when thedeposition amount of PM between the electrodes 551 and 552 is apredetermined second determination PM deposition amount Qpme2 that isless than a first determination PM deposition amount Qpme1. When theoutput value of the PM sensor 55 reaches the determination output valueSout0 in a time period from the voltage applying time to the firstdetermination time td1 as shown in FIG. 8, the time at which the outputvalue of the PM sensor 55 reaches the determination output value Sout0is defined as the second determination time td2.

In the configuration that this time is defined as the seconddetermination time td2, there is a difference between the sensor outputchange rates from the voltage applying time to the second determinationtime td2 in the case where the output value of the PM sensor 55 isincreased by gradually increasing the deposition amount of PM betweenthe electrodes 551 and 562 and in the case where the output value of thePM sensor 55 is increased by trapping extraneous substances between theelectrodes 551 and 552, as shown in the lower graph of FIG. 8. Morespecifically, in the case where the output value of the PM sensor 55 isincreased by trapping extraneous substances between the electrodes 551and 552, as shown by the curve L6 in the lower graph of FIG. 8, there isa time duration with a relatively high sensor output change rate in thetime period from the voltage applying time to the second determinationtime td2. In the case where the output value of the PM sensor 55 isincreased by gradually increasing the deposition amount of PM betweenthe electrodes 551 and 552, on the other hand, as shown by the curve L5in the lower graph of FIG. 8, the sensor output change rate has arelatively small variation in the time period from the voltage applyingtime to the second determination time td2. It is thus distinguishablewhether the increase in output value of the PM sensor 55 is to beattributed to the gradual increase in deposition amount of PM betweenthe electrodes 551 and 552 or is to be attributed to the extraneoussubstances trapped between the electrodes 551 and 552, with highaccuracy based on the sensor output change rate in the time period fromthe voltage applying time to the second determination time td2.

In the case where the output value of the PM sensor 55 reaches thedetermination output value Sout0 by gradually increasing the depositionamount of PM between the electrodes 551 and 552, the deposition amountof PM between the electrodes 551 and 552 is relatively large after that(after the second determination time td2). This results in increasingthe sensor output change rate as shown by the curve L5 in the lowergraph of FIG. 8. After the second determination time td2, it isaccordingly difficult to distinguish whether the increase in outputvalue of the PM sensor 55 is to be attributed to the gradual increase indeposition amount of PM between the electrodes 551 and 552 or is to beattributed to the extraneous substances trapped between the electrodes551 and 552, with high accuracy based on the sensor output change rate.

According to this embodiment, it is also determined that the filterdiagnosis process is not to be performed in the case where the sensoroutput change rate becomes higher than the determination change rate Rthin the time period from the voltage applying time to the seconddetermination time td2. In the case where the sensor output change ratedoes not become higher than the determination change rate Rth in thetime period from the voltage applying time to the second determinationtime td2, on the other hand, the filter diagnosis process is to beperformed based on the output value of the PM sensor 55 at the firstdetermination time td1, irrespective of the second output change rateafter the second determination time td2. This results in improving theaccuracy of abnormality diagnosis of the filter 51 using the outputvalue of the PM sensor 55. This also suppresses execution of abnormalitydiagnosis of the filter 51 from being unnecessarily prohibited.

In some cases, the output value of the PM sensor 55 may not reach thedetermination output value Sout0 in the time period from the voltageapplying time to the first determination time td1. In such cases,however, the output value of the PM sensor 55 at the first determinationtime td1 is inevitably smaller than the abnormality determination valueSth. Accordingly, it is determined that the filter 51 is normal,irrespective of the sensor output change rate in the time period fromthe voltage applying time to the first determination time td1.

The following describes a flow of filter abnormality diagnosis of thisembodiment with reference to FIG. 9. FIG. 9 is a flowchart showing theflow of filter abnormality diagnosis of the embodiment. This flow isperformed by the ECU 10 on satisfaction of predetermined filterdiagnosis preliminary conditions. In an application that the PM sensor55 is provided with an SCU, this flow may be performed by the SCU. Thesteps in this flow at which the like processes to those of the steps ofFIG. 6 are performed are expressed by the like step numbers and are notspecifically described.

In the case of a negative answer at S105, the flow proceeds to theprocess of S306. At S306, the flow determines whether the current outputvalue Sout of the PM sensor 55 is equal to or larger than apredetermined determination output value Sout0. This determines whetherthe time comes to the second determination time td2. In the case of anaffirmative answer at S306, it is determined that the output value Soutof the PM sensor 55 has reached the determination output value Sout0without setting the diagnosis prohibition flag ON in the determinationprocess of S104 after the voltage applying time. In other words, it isdetermined that substantially no extraneous substances are trappedbetween the electrodes 551 and 552 of the PM sensor 55 in the timeperiod from the voltage applying time to the second determination timetd2. Accordingly, in this case, it is determined that the filterdiagnosis process is to be performed based on the output value of the PMsensor 55 at the first determination time td1. The flow subsequentlyproceeds to the process of S107.

In the case of a negative answer at S306, i.e., when the output valueSout of the PM sensor 55 has not yet reached the determination outputvalue Sout0, on the other hand, the flow proceeds to the process ofS307. At S307, the flow determines whether the current referencedeposition amount of PM Qpme is equal to or larger than the firstdetermination PM deposition amount Qpme1, like S107. This determineswhether the time comes to the first determination time td1 after elapseof the determination time period dtd since the voltage applying time. Inthe case of an affirmative answer at S307, the time comes to the firstdetermination time td1 while the output value Sout of the PM sensor 55has not yet reached the determination output value Sout0. Accordingly,in this case, the flow subsequently determines that the filter 51 has noabnormality but is normal at S110. In the case of a negative answer atS307, i.e., when the reference deposition amount of PM Qpme has not yetreached the first determination PM deposition amount Qpme1, on the otherhand, the determination process is performed again at S104.

The above flow of filter abnormality diagnosis determines that thefilter diagnosis process is not to be performed, in the case whereextraneous substances are trapped between the electrodes 551 and 552 ofthe PM sensor 55 in the time period from the voltage applying time tothe second determination time td2 before the first determination timetd1. The flow of filter abnormality diagnosis determines that the filterdiagnosis process is to be performed irrespective of the sensor outputchange rate after the second determination time td2, on the other hand,in the case where substantially no extraneous substances are trappedbetween the electrodes 551 and 552 of the PM sensor 55 in the timeperiod from the voltage applying time to the second determination timetd2 before the first determination time td1.

According to embodiments of the present disclosure, the PM sensor 55 isconfigured to generate the output value that reflects the value ofelectric current flowing between the electrodes 551 and 552. The outputvalue of the PM sensor 55 accordingly increases with an increase indeposition amount of PM between the electrodes 551 and 552. According toa modification, the PM sensor 55 may be configured to generate an outputvalue that reflects the resistance value between the electrodes 551 and552. In this modification, the output value of the PM sensor 55decreases with an increase in deposition amount of PM between theelectrodes 551 and 552. In this modification, at S203 in the flow ofdetermination process shown in FIG. 7 to determine whether execution ofthe filter diagnosis process is to be prohibited or not, the sensoroutput change rate Rsout is calculated according to Equation (2) givenbelow:

Rsout=(Sout1−Sout2)/(Qpme2·Qpme1)  (2)

In the PM sensor 55 configured to have the output characteristic thatthe output value of the PM sensor 55 decreases with an increase indeposition amount of PM between the electrodes 551 and 552, the largerdeposition amount of PM between the electrodes 551 and 552 provides thehigher rate of decrease in output value of the PM sensor 55 relative tothe increase in deposition amount of PM. Accordingly, in thismodification, the sensor output change rate gradually increases with anincrease in reference deposition amount of PM even when substantially noextraneous substances are trapped between the electrodes 551 and 552.

According to embodiments of the present disclosure, as shown in thelower graphs of FIGS. 5 and 8, the determination change rate Rth is setto the fixed value irrespective of the reference deposition amount ofPM. According to a modification, the determination change rate Rth maybe set to increase gradually or stepwise with an increase in referencedeposition amount of PM. Setting the determination change rate Rth inthis way makes the sensor output change rate unlikely to exceed thedetermination change rate Rth even when the sensor output change rategradually increases with an increase in deposition amount of PM betweenthe electrodes 551 and 552. Even in this modification, however, at atime close to the first determination time td1, a large amount of PMdeposits between the electrodes 551 and 552, so that the sensor outputchange rate in the case where the output value of the PM sensor 55 isvaried by increasing the deposition amount of PM between the electrodes551 and 552 is likely to exceed the determination change rate Rth. Amodification of this procedure thus determines whether the filterdiagnosis process is to be performed or not, based on the sensor outputchange rate in the time period from the voltage applying time to thesecond determination time td2, as described above.

According to embodiments of the present disclosure, the sensor outputchange rate Rsout is calculated as the parameter used for filterabnormality diagnosis. According to a modification, a value correlatedwith the sensor output change rate may be used instead as the parameter.For example, a differential between a first output difference and asecond output difference or a ratio of the first output difference tothe second output difference may be used as the parameter of abnormalitydiagnosis. The first output difference denotes a difference between thefirst determination PM deposition amount Qpme1 and the first outputvalue Sout1 of the PM sensor 55, which are used for calculation of thesensor output change rate Rsout in the flow of determination processshown in FIG. 7. The second output difference denotes a differencebetween the second determination PM deposition amount Qpme2 and thesecond output value Sout2 of the PM sensor 55, which are used forcalculation of the sensor output change rate Rsout.

According to embodiments of the present disclosure, the referencedeposition amount of PM is the estimated value on the assumption thatthe filter 51 is in the reference failure state. According to amodification, the reference deposition amount of PM may be an estimatedvalue on the assumption that the filter 51 is not provided in theexhaust passage 5. In this modification, the first determination PMdeposition amount used to specify the determination time period dtd andthe abnormality determination value to be compared with the output valueof the PM sensor 55 in the filter diagnosis process may be set on thepremise that the reference deposition amount of PM is the estimatedvalue on the assumption that the filter 51 is not provided in theexhaust passage 5.

According to embodiments of the present disclosure, as shown in FIG. 10,a differential pressure sensor 56 may be provided in the exhaust passage5. The differential pressure sensor 56 outputs an electric signalcorresponding to a difference in exhaust pressure between upstream anddownstream of the filter 51. Like the output signals of the othersensors, the output signal of the differential pressure sensor 56 isinput into the ECU 10. At the time of completion of the filter recoveryprocess, i.e., in the state that substantially no PM deposits on thefilter 51, the output value of the differential pressure sensor 56 iscorrelated with the state of the filter 51 to some extent. Morespecifically, in the fixed operating conditions of the internalcombustion engine 1, i.e., at the fixed flow rate of exhaust gas flowinginto the filter 51, the difference in exhaust pressure between upstreamand downstream of the filter 51 decreases with an increase in degree ofdeterioration of the filter 51. The higher degree of deterioration ofthe filter 51 accordingly provides the smaller output value of thedifferential pressure sensor 56. In the configuration that thedifferential pressure sensor 56 is provided in the exhaust passage 5,the state of the filter 51 may be estimated, based on the output valueof the differential pressure sensor 56 at the time of completion of thefilter recovery process. It may be, however, difficult to estimate thestate of the filter 51 with high accuracy using only the output value ofthe differential pressure sensor 56. In the configuration with thedifferential pressure sensor 56, there may be a need for filterabnormality diagnosis using the output value of the PM sensor asdescribed in embodiments of the present disclosure.

In the configuration of FIG. 10, the state of the filter 51 may beestimated, based on the output value of the differential pressure sensor56 at the time of completion of the filter recovery process. Adeposition amount of PM between the electrodes 551 and 552 of the PMsensor 55 that is estimated on the assumption that the filter 51 is inthis estimated state defined as a reference state may be specified asthe reference deposition amount of PM. This allows for estimation of thereference deposition amount of PM on the assumption that the state ofthe filter 51 is close to the actual state to some extent. This providesthe higher correlation between the reference deposition amount of PM andthe output value of the PM sensor 55 in the case where the output valueof the PM sensor 55 is varied by gradually increasing the depositionamount of PM between the electrodes 551 and 552 of the PM sensor 55.This provides a more noticeable difference between the variation inoutput value of the PM sensor 55 attributed to the gradual increase ofthe deposition amount of PM between the electrodes 551 and 552 and thevariation in output value of the PM sensor 55 attributed to theextraneous substances trapped between the electrodes 551 and 552.Accordingly, this may increase the accuracy of differentiation based ontheir sensor output change rates as described above in embodiments ofthe present disclosure.

As described above, in general, the reference deposition amount of PMgradually increases with time after the voltage applying time. When theabscissa is changed to the time elapsed since the voltage applying timein the upper graphs of FIGS. 5 and 8, the output value of the PM sensor55 is expected to vary in the same tendencies as those shown in FIGS. 5and 8. In Embodiments 1 and 2 described above, the variation in outputvalue of the PM sensor 55 per unit time may thus be specified as thesensor output change rate. In this modification, the sensor outputchange rate at the time when extraneous substances are trapped betweenthe electrodes 551 and 552 of the PM sensor 55 also becomes higher thanthe sensor output change rate in the case where the deposition amount ofPM between the electrodes 551 and 552 is gradually increased.Accordingly, this sensor output change rate may be used similarly to thevariation in output value of the PM sensor 55 per unit increase of thereference deposition amount of PM described above.

While the present disclosure has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

REFERENCE SIGNS LIST

-   1 internal combustion engine-   4 intake passage-   5 exhaust passage-   50 oxidation catalyst-   51 particulate filter (filter)-   55 PM sensor-   550 insulator-   551, 552 electrodes-   553 sensor element-   554 ammeter-   555 heater-   556 cover-   557 through hole-   56 differential pressure sensor-   60 power source-   10 ECU

What is claimed is:
 1. An abnormality diagnosis apparatus for aparticulate filter, the abnormality diagnosis apparatus comprising: aparticulate matter (PM) sensor comprising a pair of electrodes, the PMsensor being configured to output a value corresponding to a depositionamount of PM between the pair of electrodes; a controller comprising atleast one processor configured to perform a sensor recovery process anda filter diagnosis process, the sensor recovery process comprising aprocess of removing PM depositing between the electrodes of the PMsensor, and the filter diagnosis process comprising a process ofdiagnosing an abnormality of the particulate filter based on an outputvalue of the PM sensor at a predetermined first determination time aftera predetermined PM deposition restart time; and a monitor unit that isconfigured to monitor an output value of the PM sensor after the PMdeposition restart time.
 2. The abnormality diagnosis apparatusaccording to claim 1, wherein an estimated value of the depositionamount of PM between the electrodes of the PM sensor during an assumedpredetermined reference state is specified as a reference depositionamount of PM.
 3. The abnormality diagnosis apparatus according to claim2, wherein a variation in an output value of the PM sensor monitored bythe monitor unit per unit increase in the reference deposition amount ofPM, or a variation in output value of the PM sensor monitored by themonitor unit per unit time, is specified as a sensor output change rate4. The abnormality diagnosis apparatus according to claim 3, whereinwhen the sensor output change rate becomes higher than a predetermineddetermination change rate in a time period from the PM depositionrestart time to a predetermined second determination time prior to thepredetermined first determination time, the filter diagnosis process bythe controller is not performed; and when the sensor output change ratedoes not become higher than the predetermined determination change ratein the time period from the PM deposition restart time to thepredetermined second determination time, the filter diagnosis process bythe controller is performed irrespective of the sensor output changerate after the predetermined second determination time.
 5. Theabnormality diagnosis apparatus according to claim 2, wherein thepredetermined first determination time is a time when the referencedeposition amount of PM reaches a predetermined first determination PMdeposition amount, and the predetermined second determination time is atime when the reference deposition amount of PM reaches a predeterminedsecond determination PM deposition amount, wherein the seconddetermination PM deposition amount is less than the first determinationPM deposition amount.
 6. The abnormality diagnosis apparatus accordingto claim 2, wherein the predetermined first determination time is a timewhen the reference deposition amount of PM reaches a predetermined firstdetermination PM deposition amount, an output value of the PM sensor,when the deposition amount of PM between the electrodes is apredetermined second determination PM deposition amount that is lessthan the predetermined first determination PM deposition amount, isspecified as a determination output value, and a time, in a time periodfrom the PM deposition restart time to the predetermined firstdetermination time, when the output value of the PM sensor reaches thedetermination output value, is specified as the predetermined seconddetermination time.
 7. The abnormality diagnosis apparatus according toclaim 3, wherein the sensor output change rate is a variation in anoutput value of the PM sensor monitored by the monitor unit per unitincrease of the reference deposition amount of PM.
 8. The abnormalitydiagnosis apparatus according to claim 1, further comprising: adifferential pressure sensor configured to output a value correspondingto a difference in exhaust pressure upstream and downstream of theparticulate filter in an exhaust passage, wherein the controller isfurther configured to perform a filter recovery process, the filterrecovery process comprising a process of removing PM depositing on theparticulate filter, and wherein a state of the particulate filter isestimated based on an output value of the differential pressure sensorat a time of completion of the filter recovery process performed by thecontroller prior to execution of the sensor recovery process by thecontroller, and the reference deposition amount of PM is estimated onthe assumption that the particulate filter is in an estimated statespecified as a reference state.
 9. The abnormality diagnosis apparatusaccording to claim 1, wherein the PM sensor is configured such that alarger deposition amount of PM between the electrodes provides a highervariation in output value of the PM sensor relative to an increase indeposition amount of PM between the electrodes.
 10. The abnormalitydiagnosis apparatus according to claim 1, wherein the predetermined PMdeposition restart time is a time when deposition of PM between theelectrodes of the PM sensor is restarted after completion of the sensorrecovery process.
 11. An abnormality diagnosis method for a particulatefilter, the method comprising: outputting a value corresponding to adeposition amount of PM between a pair of electrodes of a particulatematter (PM) sensor; performing, by a controller comprising at least oneprocessor, a sensor recovery process and a filter diagnosis process, thesensor recovery process comprising a process of removing PM depositingbetween the electrodes of the PM sensor, and the filter diagnosisprocess comprising a process of diagnosing an abnormality of theparticulate filter based on an output value of the PM sensor at apredetermined first determination time after a predetermined PMdeposition restart time; and monitoring an output value of the PMsensor, by a monitor unit, after the PM deposition restart time.
 12. Theabnormality diagnosis method according to claim 11, wherein an estimatedvalue of the deposition amount of PM between the electrodes of the PMsensor during an assumed predetermined reference state is specified as areference deposition amount of PM.
 13. The abnormality diagnosis methodaccording to claim 12, wherein a variation in an output value of the PMsensor monitored by the monitor unit per unit increase in the referencedeposition amount of PM, or a variation in output value of the PM sensormonitored by the monitor unit per unit time, is specified as a sensoroutput change rate
 14. The abnormality diagnosis method according toclaim 13, wherein when the sensor output change rate becomes higher thana predetermined determination change rate in a time period from the PMdeposition restart time to a predetermined second determination timeprior to the predetermined first determination time, the filterdiagnosis process by the controller is not performed; and when thesensor output change rate does not become higher than the predetermineddetermination change rate in the time period from the PM depositionrestart time to the predetermined second determination time, the filterdiagnosis process by the controller is performed irrespective of thesensor output change rate after the second determination time.
 15. Theabnormality diagnosis method according to claim 12, wherein thepredetermined first determination time is a time when the referencedeposition amount of PM reaches a predetermined first determination PMdeposition amount, and the predetermined second determination time is atime when the reference deposition amount of PM reaches a predeterminedsecond determination PM deposition amount, wherein the predeterminedsecond determination PM deposition amount is less than the predeterminedfirst determination PM deposition amount.
 16. The abnormality diagnosismethod according to claim 12, wherein the predetermined firstdetermination time is a time when the reference deposition amount of PMreaches a predetermined first determination PM deposition amount, anoutput value of the PM sensor, when the deposition amount of PM betweenthe electrodes is a predetermined second determination PM depositionamount that is less than the predetermined first determination PMdeposition amount, is specified as a determination output value, and atime, in a time period from the PM deposition restart time to the firstdetermination time, when the output value of the PM sensor reaches thedetermination output value, is specified as the predetermined seconddetermination time.
 17. The abnormality diagnosis method according toclaim 13, wherein the sensor output change rate is a variation in anoutput value of the PM sensor monitored by the monitor unit per unitincrease of the reference deposition amount of PM.
 18. The abnormalitydiagnosis method according to claim 11, further comprising: outputting,by a differential pressure sensor, a value corresponding to a differencein exhaust pressure upstream and downstream of the particulate filter inan exhaust passage, performing, by the controller, a filter recoveryprocess, the filter recovery process comprising a process of removing PMdepositing on the particulate filter, and wherein a state of theparticulate filter is estimated based on an output value of thedifferential pressure sensor at a time of completion of the filterrecovery process performed by the controller prior to execution of thesensor recovery process by the controller, and the reference depositionamount of PM is estimated on the assumption that the particulate filteris in an estimated state specified as a reference state.
 19. Theabnormality diagnosis method according to claim 11, wherein a largerdeposition amount of PM between the electrodes of the PM sensor providesa higher variation in output value of the PM sensor relative to anincrease in deposition amount of PM between the electrodes.
 20. Theabnormality diagnosis method according to claim 11, wherein thepredetermined PM deposition restart time is a time when deposition of PMbetween the electrodes of the PM sensor is restarted after completion ofthe sensor recovery process.